The present invention relates to enclosed switchboards and more particularly to devices for facilitating measurement of insulation resistance of a load associated with the enclosed switchboards and a main circuit connected to the load.
In conventional enclosed switchboards, measurement of insulation resistance of a load associated with the main switchboard and a main circuit connected to the load is difficult and dangerous.
As shown in FIG. 1 of the accompanying drawings, a conventional enclosed switchboard 10 comprises a housing 11. In the rear portion of the space inside of the housing 11, there are provided bus bars 12 electrically connected to a source of power supply, not shown, and cables 13 electrically connected to a load shown as a motor 17. A first stationary contact assembly 15 includes stationary contacts (not shown) which are electrically connected to associated ones of the bus bars 12 respectively. A second stationary contact assembly 16 includes stationary contacts 21 which are electrically connected to the associated ones of the cables 13 through conductors 18. A circuit breaker 14 of the drawout type is movable between a disconnected position in which, as illustrated, the movable contact assemblies 14a and 14b mounted on the circuit breaker are out of engagement with the stationary contact assemblies 15 and 16, and a connected position in which the movable contact assemblies 14a and 14b are in engagement with the stationary contact assemblies 15 and 16.
To measure insulation resistance, branch conductors 19 with no insulation thereon are provided in electrical connection with the conductors 18, and one terminal of an insulation resistance tester 22 is contacted with one of the branch conductors 19, with the aid of an insulating hook rod 20 which is inserted through an opening 11a provided in the rear wall of the housing 11. However, while the operator is testing at the back of the switchboard 10, the circuit breaker 14 may be erroneously brought into the connected position to render the branch conductor 19 alive, which is hazardous to the operator and which may damage the tester 22. Also, it is inconvenient that the operator has to be at the back of the switchboard 10 to carry out the measurement.
In another method of measuring the insulation resistance, one terminal of the insulation resistance tester 22 is contacted with the contact 21 of the stationary contact assembly 16. With this method, the operator can be in front of the switchboard, but, the circuit breaker 14 has to be moved out so far as not to be obstructive to the measurement. Also, it is possible that the terminal of the tester 22 is erroneously contacted with the live contact of the stationary contact assembly 15 instead of the contact 21 of the contact assembly 16.
Moreover, in certain types of enclosed switchboards (such as G-type according to the Standards of the Japan Electrical Manufacturers' Association 1153) the conductors of the main circuit must be provided with full insulation thereon. In such cases, the measurement of insulation resistance is accompanied with further difficulty.